If you read back what you've written objectively, the first part and the second part don't go together.
He had an affair. The rest of his reasoning is largely irrelevant, not all successful people start having an affair, it doesn't matter how old she was. He had an affair. You found out, he ended it, he said and did the right things to show that he wanted to be with you and it worked, you rebuilt.
Is he in a new job again now, or the same one from last year? Either way, He's got a lovely assistant again, and again, he's failed to build appropriate boundaries. A drunk colleague has mentioned this - it's more likely that she intended this to be a friendly warning that people have noticed them getting close than that she actually thinks the assistant has feelings for him and he didn't know, but either way.
Most people, at that point, would build better boundaries. Even if the boundaries were good in the first place, you'd reinforce a bit. Drop contact to strictly necessary levels, maybe keep conversations more work-related than they were before, make sure they limit alone time. Some people would mention their family more.
He didn't. He asked her to go for a drink on their own - big green light to someone who fancies him, yes? And then asks her if she fancies him. Everyone under the sun would deny it at that point I think, it's a horrendously awkward question. He then overshared that nothing could happen because he's previously had an affair... which is weird. Firstly because "Nothing can happen between us" is usually what star-crossed lovers say to each other before embarking on their tragic affairs, but because it suggests he wants it too, but it can't. And secondly because the revelation of the affair shows he has no willpower. He has no qualms with cheating, he's done it before. It almost undermines what he just said. It would be like saying "We can't steal this...but I stole one last year", or "We can't sneak in there.... but I did sneak in last year."
I suspect he told you because he thought it would get back to you that they "disappeared" together for a drink.
Maybe last year was a very good act and he wasn't all that devastated. Maybe he was very devastated, but over time he's learnt that now he's picked the pieces up once, he could do it again. Maybe he thinks his success and charms mean you could never leave him. Who knows. It's not the behaviour of someone who had an affair, regretted it and went to great lengths to stop it happening again, though. At the very best, he's flirting with danger.
Although I think Costa is right and at least he's pretty much guaranteed that she won't have feelings for him anymore.